1 BitTorrent System Efrat Oune Bar-Ilan 2005

2 What is BitTorrent? BitTorrent is a peer-to-peer file distribution system (built for intensive daily use by a very large user community) Good P2P design is : Decentralized computing Every peer acts as a client and a server

3 The key philosophy of BitTorrent Problem: more customers require more bandwidth The BitTorrent Solution: customers help distribute content, they should upload at the same time they are downloading In this manner, network bandwidth is utilized as efficiently as possible.

4 The purpose of BitTorrent BitTorrent developed for transfering big files (700 Mb and up) at minimal time such as Linux images, video files etc, and the download rates is the best as the file request popularity increase!!! Therefore BitTorrent specializes in downloading fast new files such as newly released TV shows, films,DVD rips, application.

5 The unique in BitTorrent Redistributes cost of upload to downloaders (thus making 1. hosting a file with a potentially unlimited number of downloaders affordable 2.downlowds large files) Suitable for file swarming (a lot of people are trying to download the same copy simultaneously) Has best connections to the missing pieces (has more the 7 methods to get the files download efficiently ) Uses a Tit For Tat technique for selection of peers ( encourage downloaders to upload)

6 What the user should do? Find a.torrent file of interest (any Web site can host this) Download it BitTorrent does the rest… A peer contacts the tracker to find other peers Peers contact one another and share information

7 But What is Really Going On? BitTorrent breaks up large files into pieces, then allows users to swarm - exchanging small portions of files with one another in a tit-for-tat-like manner (technique)- until everyone has a complete set. A peer first downloads a random piece of the file and then attempts to find rare pieces As soon as the client finishes importing a block, it hashes it to make sure that the block matches what the torrent file said it should be

8 Terms - Seeds and Leech Seed: a peer which after finishing download a file, staying online for a while and sharing the file for free, i.e., without bartering Leech: a peer which trying to download the file without uploading it to anyone.

9 The feature of seed If all seeds get taken offline, the file may no longer be available for download, even if the torrent file is possessed. However, even if there are no seeds, as long as there is at least one distributed copy of the file everyone can eventually get the complete file.

10 Measurement features: Download performance Integrity Flashcrowds Availability (Technical analyses of BitTorrent, Johan Pouwelse, A detailed study of the BitTorrent network)Technical analyses of BitTorrent

11 Download performance The more you share, the faster your downloads. The more people trading a file, the more options for obtaining its pieces.

12 Availability The availability of content is unpredictable. When the popularity drops and the last peer/seed with certain content goes offline, the content dies Popularity downloads availability

13 Integrity a.torrent file saves a hash of each block in the file (which allows users to make sure they are downloading the real thing). The BitTorrent system seems to be very effective in removing fake and corrupted files.

14 Flashcrowds BitTorrent focus on the system's reaction to the sudden popularity of a single file. BitTorrent components are capable of efficiently handling very large flashcrowds. It be designated for it.

15 BitTorrent developer claims For very popular files, BitTorrent can support about a thousand times as many downloads as HTTPHTTP BitTorrent achieves a higher level of robustness and resource utilization than any currently known cooperative technique

16 Studies found : only very few P2P system have stood the test of intensive daily use The BitTorrent file-sharing system is one of these systems and most of popular networks In fact, BitTorrent traffic made up 53 per cent of all P2P traffic in June (Johan Pouwelse, 18th December, 2004 )

17 Advantages/Strengths Better bandwidth utilization Limit free riding – tit-for-tat Limit leech attack – coupling upload & download Spurious files not propagated Ability to resume a download Open Source implementations

18 Drawbacks Scalability Millions of peers – Tracker behavior uses 1/1000 of bandwidth The tracker limits scalability and creates a single point of failure Robustness System progress dependent on altruistic nature of seeds and peers BitTorrent the availability of content is unpredictable

19 Compare to Napster In Napster every peer is also a client and server BitTorrent is like napster but uses file swarming Unlike the old Napster, popularity doesn't bog down the process -- it gives it a shot of adrenaline

20 Compare to Emule Emule has a credits system whereby a client stores the amount of data it has uploaded and downloaded from every client; and gives clients that have best uploaded to it a higher priority in the queue. The BitTorrent is "leech resistance" by design.

21 Compare to eDonkey2000 network The original eDonkey2000 client provided little "leech resistance“; Most new eDonkey clients have some sort of system to encourage uploaders The method used by BitTorrent to distribute files parallels to a large extent the one used by the eDonkey2000 network, but nodes in eDonkey's file sharing network usually share and download a much larger number of files, making the bandwidth available to each transfer much smaller.

22 Scenario1 Web page with link to.torrent A B C Peer [Leech] Downloader “US” Peer [Seed] Peer [Leech] Tracker Web Server.torrent

23 Scenario2 Web page with link to.torrent A B C Peer [Leech] Downloader “US” Peer [Seed] Peer [Leech] Tracker Get- announce Web Server

24 Scenario3 Web page with link to.torrent A B C Peer [Leech] Downloader “US” Peer [Seed] Peer [Leech] Tracker Response-peer list Web Server

25 Scenario4 Web page with link to.torrent A B C Peer [Leech] Downloader “US” Peer [Seed] Peer [Leech] Tracker Shake- hand Web Server Shake- hand

26 Scenario5 Web page with link to.torrent A B C Peer [Leech] Downloader “US” Peer [Seed] Peer [Leech] Tracker pieces Web Server

27 Scenario6 Web page with link to.torrent A B C Peer [Leech] Downloader “US” Peer [Seed] Peer [Leech] Tracker pieces Web Server

28 Scenario7-all scenario Web page with link to.torrent A B C Peer [Leech] Downloader “US” Peer [Seed] Peer [Leech] Tracker Get-announce Response-peer list pieces Web Server

29 Links The official BitTorrent Home Page BitTorrent- Wikipedia,the free encyclopedia Technical analyses of BitTorrent Interview with the creator of BitTorrent List of BitTorrent clients List of web for Torrent Files